This invention relates to a device for optical measurement of voltage and electric field by making use of an electro-optic crystal.
Fundamentally, a device for optical measurement of voltage and electric field by making use of an electro-optic crystal operates by measuring a voltage which is applied to the electro-optic crystal. Such a device may be used as a voltage measuring device if electrodes connected to both surfaces of the electro-optic crystal are connected directly to terminals across which a voltage to be measured is applied, and can be used as an electric field measuring device if it is placed in an electric field to be measured with the electrodes removed.
An example wherein the foregoing device is used as a voltage measuring device is shown in FIG. 1(a). The voltage measuring device includes a polarizer 2, a quarter-wave plate 3, an electro-optic crystal 4 and an analyzer 5 arranged in the stated order in the direction of advancement of light from a light source 1. A pair of transparent electrodes 6 (of which only one is visible in the drawing) are affixed to the electro-optic crystal 4, one on each of the opposing sides, and are supplied by a voltage source 7 with a voltage to be measured. Meanwhile, the polarizer 2 converts the light from the light source 1 into linearly polarized light, which is in turn converted into circularly polarized light by the quarter-wave plate 3. The electro-optic crystal 4 subjects the circularly polarized light to phase modulation to effect a conversion into elliptically polarized light. If we let the indices of refraction of the electro-optic crystal 4 be denoted by n.sub.x, n.sub.y when the voltage to be measured is at zero, then applying a voltage of V volts will cause these indices to change to n.sub.x -KV, n.sub.y +KV, respectively, where n.sub.x is the refractive index for linear polarization in the X-direction, n.sub.y is the refractive index for linear polarization in the Y-direction, and K is a constant. Since the refractive indices of the crystal with respect to the polarized light differ in the X and Y directions, that is, since there is a difference in the speed of light along the X- and Y-direction vector components of the circularly polarized light, the light which has been circularly polarized by the quarter-wave plate 3 is converted into elliptically polarized light owing to a phase conversion effected by passage through the electro-optic crystal 4.
In the conventional apparatus, the plane of polarization of the analyzer 5 is set at an angle of 45.degree. with respect to the optical axis of the electro-optic crystal, with the elliptically polarized light being subjected to amplitude modulation. The reason for the inclination of 45.degree. is to maximize sensitivity and improve linearity, as will be described later with reference to FIG. 1(b).
If we let the power of the light incident upon the polarizer 2 be represented by P.sub.in and the measuring section loss be represented by l', then the relation between the power P.sub.out of the light delivered by the analyzer 5 and a voltage V.sub.in to be measured will be expressed by the following equation in the absence of the quarter-wave plate 3: ##EQU1## where V.sub..pi. is the half-wave voltage which depends upon the type of electro-optic crystal and the orientation of its optical axis when in use. In order to employ a region of the characteristic curve of Eq. (1) that has good linearity, it suffices to insert the quarter-wave plate 3, which serves as optical biasing means, between the polarizer 2 and electro-optic crystal 4, or between the crystal and the analyzer 5. Such an expedient enables the selection of a region near the point .lambda./4 where the curve has good linearity, as depicted in FIG. 1(b). With the quarter-wave plate 3 inserted, P.sub.out is given by the following equation: ##EQU2## The foregoing may be rewritten as follows in the range ##EQU3## The optical signal output of the analyzer is converted into an electric signal through use of an element such as a PIN photo-diode, thereby enabling determination of the voltage V.sub.in.
Illustrated in FIG. 1(c) is an example of another known arrangement for supplying the electro-optic crystal 4 with a voltage to be measured. This arrangement serves the same purpose as the arrangement of FIG. 1(a), as will be described below.
Voltage measurement carried out in accordance with the foregoing principles is well-known in the art, in which crystals of KDP, ADP, LiNbO.sub.3 and LiTaO.sub.3, etc., are employed as the electro-optic crystal 4. The use of these crystals results in the same disadvantage, however, namely a poor temperature characteristic exhibited by the measuring device relying upon them. More specifically, the characteristic is such that the sensitivity of the device fluctuates with temperature. Each of the aforementioned crystals has refractive indices n.sub.x, n.sub.y that are slightly different from each other, and the refractive indices n.sub.x, n.sub.y exhibit different temperature characteristics. In other words, such crystals have a natural birefringence, as may be expressed by n.sub.x =n.sub.o -KV, n.sub.y =n.sub.e +KV, with the indices of birefringence along the respective main axes having mutually different temperature characteristics. This is the cause of the afore-mentioned poor temperature characteristic possessed by the measuring device.
In order to compensate for this instability with respect to temperature change, a so-called "temperature compensation-type" arrangement, shown in FIG. 2, has been proposed in which two crystals A and B are coupled together with the orientations of their optical axes displaced from each other by 90 degrees. If the crystals A and B are precisely machined so that the length l.sub.1 of crystal A is exactly equal to the length l.sub.2 of crystal B, and if the crystals are then joined with their optical axes displaced correctly by 90 degrees, this will eliminate a term, in a theoretical equation expressing P.sub.out, which includes the difference between the refractive index n.sub.o of an ordinary ray and the refractive index n.sub.e of an extraordinary ray. In theory, therefore, this should make it possible to effect a certain degree of improvement in the temperature characteristic. In actual practice, however, it is extremely difficult to machine two crystals to exactly the same length and join them together with their optical axes meeting at 90 degrees. The result is a temperature compensation-type apparatus that is high in cost.
FIG. 3 shows an example of a temperature characteristic exhibited by a temperature compensation-type voltage measureing device using a crystal of LiNbO.sub.3. The relative degree of modulation (m) when a voltage (V.sub.in) of constant magnitude is applied to the crystal is plotted along the vertical axis as a function of temperature. Where the relative degree of modulation (m) is defined by the following equation: ##EQU4## Substituting Eq. (2) into the above gives: ##EQU5## This result shows that the temperature compensation is very difficult for the crystals with natural birefringence.
The application has already invented and filed (Japanese Patent Application No. 2274/80, Japanese Patent Public Disclosure No. 100364/81) a voltage and electric field measuring device in which bismuth silicon oxide (Bi.sub.12 SiO.sub.20, referred to hereinafter as BSO), or bismuth germanium oxide (Bi.sub.12 GeO.sub.20, referred to hereinafter as BGO), is used as the electro-optic crystal. Though both BSO and BGO are characterized by an optical rotatory power, the temperature stability of a voltage and electric field measuring device that uses either of these materials is excellent, being within .+-.2% in a temperature range of from -15.degree. to 60.degree. C. The fact that these materials afford such a high temperature stability indicates that they may be used to great advantage in devices for measuring voltages and electric fields in a variety of electrical systems that tend to sustain temperature fluctuations over a wide temperature range.